The use of high-energy light in ophthalmology, for example, focused sunlight, the radiation of a xenon arc lamp, or from laser beams, e.g., for coagulating retinal tissue on the fundus, is generally known. Thereby, the use of a laser as a radiation source has become particularly important because the spectral wavelength of the laser light can be exactly determined (monochromatic, optionally defined wavelengths in the green, yellow, red, or infrared spectral range) and a precise control of the laser light is possible.
Depending on the type of laser that is used, laser pulses (e.g., with the Nd:YAG laser, wavelength 1064 nm and/or 532 nm—frequency-doubled) or a continuous laser beam (continuous wave=CW laser, e.g., argon laser with wavelengths of blue=488 nm, and green=514 nm, or also the frequency-doubled Nd:YAG laser at 532 nm) are produced. As a rule, argon lasers, dye lasers and solid-state lasers are used for the coagulation of the retina in case of diabetic retinopathy, for peripheral retinal degeneration, for the treatment of retinal holes, as well as for laser trabeculoplasty for decreasing the intraocular pressure. By contrast, CO2 lasers are commonly used for the cutting of tissue.
Temperature control is particularly important for the thermal treatment of tissue. For laser coagulation, irradiation times of 20 ms to 500 ms, particularly approximately 100 ms, are commonly used for creating temperatures above 60° C. Thereby, a laser power of 100 mW to 500 mW is usually applied. With temperatures that are too low, the coagulation effect is insufficient, and with temperatures that are too high, unwanted tissue damage may occur.
A method and a device of the initially stated type, with which a control of the treatment temperature is achieved, are known from the patent document DE 101 35 944 C2. The therein disclosed treatment device exhibits a radiation source in the form of a continuously operating laser (CW laser), the laser beam of which is fed into fiber optics by use of coupling optics, directed towards irradiation optics and from there towards the eye to be treated. Thereby, the laser beam passes through a contact glass which is positioned on the eye and equipped with an acoustic or optical detector.
According to a first embodiment, an additional pulsed radiation source is provided which produces short radiation pulses at predetermined or controlled intervals. Said light pulses exhibit a characteristic which differs from the treatment radiation with regard to pulse duration and energy and are also fed as measuring radiation via the coupling optics to the fiber optics. The pulses of the measuring radiation cause a thermal tissue expansion in the eye, which depends on the temperature caused by the treatment laser and which is evaluable optoacoustically by means of the above-mentioned, e.g., piezoelectric, detectors. Thereby, the thermal expansion caused by the measuring radiation is linearly dependent on the temperature, wherein said dependence can be determined by means of a calibration measurement (Gruneisen coefficient).
According to an alternative suggestion, the additional radiation source is foregone and the treatment beam is instead interrupted for a few nanoseconds. This causes a contraction of the treated tissue on the fundus, the pressure wave of which can also be used for determining the temperature. In this case, a separate measuring beam is not provided.
Reference is expressly made to the embodiments described in the patent document DE 101 35 944 C2 with regard to the basic design and the use of laser radiation in ophthalmology as well as to detectors suitable for recording the expansion.
In the laid-open application DE 199 16 653 A1, a device for individual laser radiation dosage for the transscleral laser cyclophotocoagulation is disclosed, wherein pressure transients are produced in the target tissue of the treatment, with which a treatment plan is made in advance. Furthermore, said information can be utilized in the further course of treatment for the management of the therapy. Thereby, diagnosis pulses with low energy are produced simultaneously with the treatment radiation, which are modulated onto the treatment laser pulse or sent out beforehand. Thereby, the measuring radiation can be sent out beforehand from an additional radiation source or produced through a modulation during the coagulation irradiation with the treatment radiation source.
From the patent document DE 30 24 169 C2, a further method and a further device for operating a photocoagulator for biological tissue, particularly in ophthalmology, is known. Hereby, an arrangement is provided which measures the temporal behavior of the brightness at the point of coagulation, which is caused by the treatment radiation source or a measuring beam which is produced by an additional radiation source. Thereby, the temporal behavior of the brightness during the radiation treatment is utilized for adjusting the exposition parameters.